Lasagna

Variations

• Su böreği names this dish and states the rule. That page says "structurally it is lasagne: boiled sheets, something wet ladled between them, stacked and baked", and then: "What you put between the sheets is what decides crisp or soft." Lasagne puts two wet things in that position, and they are not doing the same job.
• The two sauces are a division of labour. The ragù is water and flavour; the béchamel is the starch that holds that water. Build it with ragù alone and there is nothing binding the liquid, so it runs the moment you cut — not for want of flavour but for want of anything to hold it. Moussaka makes the same layered dish with the béchamel alone, which is why that page puts egg yolks in it.
• Vegetarian: Roasted vegetables, spinach, and ricotta instead of meat.
• Spinach-ricotta: Use ricotta and spinach layers instead of bechamel.
• No-boil sheets: Easier — just be generous with the sauce.
• Use leftover bolognese: Build it with ragu you already have.

Tips

• Rich ragu, silky bechamel — the contrast of the two sauces is what makes lasagna.
• No-boil sheets are convenient but go dry with too little sauce — spread it generously.
• Start and end with sauce so the pasta doesn't dry out.
• A 10-15 minute rest after baking is essential — cut too soon and it runs.
• Bake covered with foil, then uncover for the last 10 minutes to brown the top.
• The rest is the béchamel setting, and moussaka says so. That page puts it as "the resting is the béchamel setting", and explains that with two egg yolks in it, egg sets on cooling as much as on heating. Lasagne's béchamel has no egg in it, so what sets is purely the starch — and leche frita gives that property in one line: a starch gel is reversible, firm when cold and loose when hot. Straight out of the oven it is a sauce; ten minutes later it holds an edge.
• A béchamel roux is barely cooked at all. As moussaka points out from thickening, a roux taken to dark brown is there for aroma and thickens almost nothing. Here thickening is the entire job, so it is held pale — just long enough to lose the raw flour taste.
• No-boil sheets want more sauce because they are drinking it. Dried pasta put in raw has to hydrate from somewhere, and all the water in this dish is in the two sauces. Mie goreng states the general case: noodles go on taking up sauce even on the plate (cooking noodles).
• Starting and ending with sauce is the same point. A sheet with a dry face has nothing to take water from, so it stays hard — and against the hot dish itself it scorches.
• The foil on, then off, is explained by croque monsieur. That page notes that while water is present a surface stalls at 100°C and essentially nothing browns (browning). Covered, the dish steams and the sheets hydrate; only in the last uncovered ten minutes does the top dry out far enough to colour. It is why that sandwich is toasted before its sauce goes on.